In the fabrication of many metallic articles for modern technology, it has been found to be advantageous to utilize powder metallurgy, either because the metals which are used cannot conveniently be shaped by earlier conventional machining techniques, or because the shapes are such that the earlier casting and machining technology is ineffective. In addition, it is possible by powder metallurgical techniques to produce bodies which can have some degree of porosity, an advantage when fluids may be taken up or dispensed by the body, or where the finished article should have a lesser density than that of the solid elemental metal or alloy.
Powder metallurgical techniques also may be used to advantage when the composition of the body is such that an equivalent composition cannot be made by casting or the like.
In general, the powder metallurgical approach requires the shaping of a mass of metallic powder and the subsequent or contemporaneous stabilization of the shaped mass so that the desired strength and shape retentivity can be achieved.
In the usual approach, a finely divided metal powder in a flowable form is poured into a cavity having the shape of the desired body and is compacted, compressed or otherwise densified in this mold to produced a so-called green compact.
The green compact is then sintered to produce a structurally stable body which may be used as such or which can be formed, densified, shaped or altered as to its physical and/or chemical properties to yield the ultimate object. Typical of the processes to which the sintered body can be subjected are various pressing and forging techniques which alter the grain size and strengthen the body.
The sintering step requires fusion of the mutually contacting metal particles of the green compact so that the particles partially merge with one another at their contact points or surfaces, the sintering temperature being below the melting point at which the particles are converted into a liquid phase.
Subsequent to or during sintering, the body can be densified by the application of pressure.
The sintered body is generally porous and can be used as such if it has sufficient strength, or can be subjected to the further densification or strengthening steps mentioned, these steps generally involving a reduction in the porosity of the body.
The further compaction after sintering can be effected in the cold state of the body or in a warm or hot condition thereof and it is also possible within the context of conventional powder metallurgy, to subject a previously sintered and densified body to a further sintering step at an elevated temperature to cause even further coalescence of the particles at their interfaces or modification of the grain structure.
The starting powder can be free from a binder or can be combined with a binder.
Naturally, the term "green" is here used in the sense in which this term has been employed in the powder metallurgical field heretofore, not to refer to an object having a green coloration, but rather to refer to the coherent compact in its presintered state as one in which there has been no significant coalescence of the mutually contacting surfaces.
As described in the International Journal of Powder Metallurgy and Powder Technology, 1975, Vol. 11, No. 3, Pages 209 to 220, the metal powder can be combined with an organic binder which, in this publication, is preferably saccharose. The binder-containing metal-powder mass can be densified in a mold of the shape of the article to be made by simple vibration.
The resulting green compact is found to have insufficient green strength to enable it to be removed from the mold and to be manipulated as desired. Hence at least partial sintering is effected while the green compact is within the mold.
The sintering of green compacts within the mold so that they can ultimately be withdrawn and handled, has significant disadvantages. Experience has shown, for example, that such sintered bodies do not have sufficiently homogenous and reproducible physical parameters such as density, strength and porosity or pore volume.
The porosity, density and strength are also adversely affected by the use of reduced metal powders which tend to interfere with sintering. The physical parameters are thus a function of the degree of sintering which can vary from body to body even where essentially identical process conditions are maintained.
In fact, these parameters can vary markedly from place to place even within a given body, especially if, in the formation of the green compact, the compacting has not been uniform.
The same applies for carbonization parameters when a carbonization is carried out in addition to the sintering. This is particularly the case when the powder metallurgical operations are carried out in the absence of binders or with small quantities of binder or the sintered body ultimately has a plurality of different cross sections.
In a preferred mode of producing sintered metal bodies, a reduced metal powder with the lowest possible oxygen content is generally employed although such metal powders are comparatively expensive. Such powders are shaped by prepressing with elevated press pressures into the green compact and sintered to produce a body which can be subsequently treated, shaped or handled.
It will thus be appreciated that earlier techniques in the fabrication by powder metallurgy of sintered metal bodies have generally involved a tradeoff whereby more expensive metal powders were required or the processing technology was more complex.
In a field remote from powder metallurgy, namely in the production of sand molds for molten metal casting, it is known to produce a mold cavity from mold sand and foundry sand binders. Such molds can be provided with so-called sand cores which also generally comprise foundry sand and binders designed to provide the core which is used to form an internal cavity hole or recess in the casting, so that the core possesses a certain degree of integrity during the casting operation but yet, as a result of the heating during the casting process, loses this integrity in whole or in part and becomes frangible or otherwise easily removable from the hole, recess or cavity once the casting is removed from the mold.
Such cores, while having sufficient green strength to resist the pressure of the molten metal during the casting operation, lose integrity upon heating during the casting process. This is also the case with more recently developed foundry sand binders of a synthetic resin base.
Sand cores of the aforedescribed type can be produced, as is well known in the foundry field, on core forming machines which have mechanized the production of such cores for foundry purposes.
Heretofore such machines have been used exclusively for the production of bodies, namely the sand cores, which intentionally have diminshed integrity and strength upon being subjected to elevated temperatures, e.g. during casting. Such machines have not, to my knowledge, ever been used in the fabrication of any bodies other than sand cores for foundry purposes and certainly have not found application in the powder metallurgical field which, by contrast with foundry applications of sand cores, requires that a heated body gain in strength as a result of the heating operation, e.g. sintering.
In fact, the production of sand cores for foundry purposes and powder metallurgical technologies are vastly different and have little if any common basis, dealing with different problems and different approaches to the solutions thereof. The field of metal shaping has long recognized a severe dichotomy between workers with the powder metallurgical approach and with foundry expertise.
From German patent documents - Printed Application DE AS 1964 426, it is known to provide sintered metal bodies using the metallic powder in combination with an organic powder in the form of a hardenable resin, generally of the epoxy type, the mixture of the powder and the resin forming a flowable mass which is poured into a cavity to produce a body analogous to a green compact which is then hardened in this cavity.
The shaped compact can then be subjected to a multi-stage heat treatment, the first stage of which involves the composition of the binder, while further stages result in sintering of the metal particles of the body together.
In this approach as with the other powder metallurgical techniques described the homogeneity or isotropy of the physical phenomena in the sintered body is not sufficient, i.e. the uniformity throughout the body leaves much to be desired and the process may not be reproducible in the fabrication in a number of such bodies which should be identical as to these physical properties.
Apparently the homogeneity and reproduceability deviates from desired levels as a result of the nonuniformity in the distribution of the metal powder in the flowable binder which, in turn, may be affected by the way in which the flowable mass is poured into the cavity or the conditions under which such pouring takes place.
In general, as to conventional powder metallurgical techniques known heretofore, a major problem has resided in the inability to produce articles of an extremely high level of homogeneity in a fully reproducible and economical manner especially when the bodies to be formed have different cross sections.